Recoil and conversion electron considerations of the ¹⁶⁶Dy/¹⁶⁶Ho in vivo generator

Abstract

The use of radionuclides as potential therapeutic radiopharmaceuticals is increasingly investigated. An important aspect is the delivery of the radionuclide to the target, i.e. the radionuclide is not lost from the chelating agent. For in vivo generators, it is not only the log K of complexation between the metal ion and the chelator that is important, but also whether the daughter radionuclide stays inside the chelator after decay of the parent radionuclide. In our previous work, we showed that the classical recoil effect is only applicable for decays with a Q value higher than 0.6 MeV (in the atomic mass range around 100). However, Zhernosekov et al. [1] published a result for¹⁴⁰Nd/¹⁴⁰Pr (Q = 0.222 MeV) which indicated that >95% of the daughter (¹⁴⁰Pr) was lost by a DOTA chelator upon decay of¹⁴⁰Nd. The authors ascribed this to the & ldquo;post-effect”. Their experiment was repeated with the¹⁶⁶Dy/¹⁶⁶Ho generator to ascertain whether our calculations were correct. It was found that 72% of the daughter (¹⁶⁶Ho) was liberated from the DOTA chelator, indicating that the “post effect” does exist in contrast to our recoil calculations. Upon further investigation, we determined that one should not only consider recoil energy levels but also the mode of decay which was able to explain the partial recoil found for¹⁶⁶Dy/¹⁶⁶Ho. It is concluded for the¹⁶⁶Dy/¹⁶⁶Ho system that the low recoil energy of the daughter nucleus¹⁶⁶Ho is not a sufficient reason to rule out release of the nuclide from chelators. On the other hand, we found that the ratio of the¹⁶⁶Ho that gets released corresponds to the ratio of relaxation of Ho atoms via the Auger process.